1. Field of the Invention
The present invention relates to an inspection device and an inspection method for evaluating characteristic changes (flaws, material property changes, etc.) in an inspection object, and in particular, to an inspection device and an inspection method suitable for the use of a magnetic inspection probe.
2. Description of the Related Art
Nondestructive testing methods employ a magnetic field. In such nondestructive testing methods, an excitation coil is energized or permanent magnets are used to generate a magnetic field. The thus-generated magnetic field is applied to a metallic material as the inspection object and then a difference in the magnetic field distribution, which is caused by flaws and/or material property changes, is detected by use of a magnetic sensor. The leakage magnetic flux method and the eddy current flaw detection method (eddy current testing method) are well-known as representative techniques of the nondestructive testing method employing a magnetic field. The leakage magnetic flux method generally employs a DC magnetic field (with no temporal change in the magnetic field intensity) or low-frequency excitation. The inspection object is infiltrated with magnetic flux therein. Magnetic flux leaking out from a part of the inspection object metal (i.e., the metallic material as the inspection object) in the vicinity of a flaw is detected by use of a magnetic sensor. In the eddy current flaw detection method, magnetic flux having a change in the magnetic flux intensity is generated by supplying a time-varying electric current (with a temporal change in the electric current value) to an excitation coil of an eddy current probe. Eddy current is caused by bringing the eddy current probe closer to the metal object (inspection object) and changes in the eddy current are detected as detection signals by a magnetic sensor.
These methods employ a magnetic sensor such as a coil or a Hall effect sensor for the detection of the magnetic field. Therefore, mechanical scanning of the probe becomes necessary for thoroughly inspecting the surface of the inspection object. In order to reduce the mechanical scanning, there has been proposed a technique for instantaneously imaging the detection signal by using an array sensor having an array of magnetic sensors. However, the spatial resolution of the obtained image varies depending on the size of the coil or Hall effect sensor used as the magnetic sensor.
Meanwhile, to further increase the resolution, a magnetic field inspection method introducing the magnetic field detection by use of light (which is called “magneto-optical effect”) is being developed. For example, there is a known probe (see JP-2006-215018-A, for example) in which the magnetic sensor is downsized by using a magnetic thin film capable of detecting the magnetic field by use of the magneto-optical effect. Further, there is a known device (see U.S. Pat. No. 5,053,704, for example) that employs a similar principle and acquires inspection signals as images by shooting a magnetic thin film with a camera. In either technique, a magnetic thin film is arranged over the inspection object surface, the magnetic thin film is irradiated with light, and the magnetic field is detected and evaluated by receiving reflected light from the magnetic thin film as response light.
The magneto-optical effect employed for these techniques is a phenomenon in which the polarization angle of light applied to magnetic material changes (rotates) depending on the status of magnetization of the magnetic material, which is well-known as the Kerr effect (for the reflected light from the object material) or the Faraday effect (for the transmitted light emerging from the object material). The amount of the rotation of the polarization angle corresponds to the sensitivity to the magnetic field, which increases with the increase in the distance of passage of the light through the object material. For this reason, the method using the transmitted light emerging from the object material (i.e., the Faraday effect) is generally employed for the nondestructive testing needing high sensitivity to the magnetic field. In this method, a reflective plate is arranged on one side of the employed magnetic thin film and the light to be used as the magnetic field information is received via an analyzer.